1. Field of the Invention
The present invention relates to an optical glass. The present invention further relates to a preform for precision press molding employing this optical glass, a method for manufacturing this preform, an optical element, and a method for manufacturing this optical element.
2. Discussion of the Background
Since the advent of digital cameras and telephones equipped with cameras, the integration and functional development of apparatuses employing optical systems have advanced rapidly. At the same time, requirements for high precision, lightweight, and compact optical systems have become ever more stringent.
To meet these requirements, optical designs employing aspherical lenses have increasingly entered the mainstream in recent years. Thus, precision press molding techniques (also known as mold pressing techniques) of directly molding optically functional surfaces by press molding without a grinding and/or polishing step have attracted attention in order to stably provide large quantities of inexpensive aspherical lenses employing high-functionality glass. The demand for optical glasses with low temperature softening properties that are suited to precision press molding is increasing each year. Such optical glasses include glasses of high refractive index and low dispersion. Patent Reference 1 describes an example of such a glass.
The glasses described in Patent Reference 1 are an optical glass characterized by comprising essential components in the form of B2O3, La2O3, Gd2O3, and ZnO; essentially comprising no lead and fluorine; and having a refractive index (nd) of 1.72 to 1.83, an Abbé number (nud) of 45 to 55, a glass transition temperature (Tg) of no higher than 630° C., and a viscosity at liquid phase temperature of not less than 0.6 Pas·s; and an optical glass characterized by comprising, denoted as molar percentages, 45 to 65 percent of B2O3, 5 to 22 percent of La2O3, 1 to 20 percent of Gd2O3 (where the combined content of La2O3 and Gd2O3 is 14 to 30 percent), 5 to 30 percent of ZnO, 0 to 10 percent of SiO2, 0 to 6.5 percent of ZrO2, and 0 to 1 percent of Sb2O3; essentially comprising no lead and fluorine; and having a refractive index (nd) of 1.72 to 1.83 and an Abbé number (nud) of 45 to 55.    Patent Reference 1: Japanese Unexamined Patent Publication (KOKAI) No. 2002-249337    Patent Reference 2: Japanese Unexamined Patent Publication (KOKAI) No. 2003-267748    Patent Reference 3: Japanese Unexamined Patent Publication (KOKAI) No. 2006-137662    Patent Reference 4: Japanese Unexamined Patent Publication (KOKAI) No. 2006-16286    Patent Reference 5: Japanese Unexamined Patent Publication (KOKAI) No. 2006-16293    Patent Reference 6: Japanese Unexamined Patent Publication (KOKAI) No. 2006-16295    Patent Reference 7: Japanese Unexamined Patent Publication (KOKAI) No. 2005-263570    Patent Reference 8: Japanese Unexamined Patent Publication (KOKAI) No. Showa 56-5345
All of the glasses described in above-cited Patent Reference 1 are so-called “high refractive index, low dispersion” optical glasses having refractive indexes (nd) of 1.72 to 1.83 and Abbé numbers (nud) of 45 to 55. However, there is a strong need to provide an optical glass having a “high refractive index and low dispersion” with a refractive index that is higher than the above-stated refractive index range and an Abbé number that is lower than the above-stated Abbé number range for the aspherical lenses employed in the digital cameras and camera-equipped telephones of recent years. Specifically, there is need for an optical glass having a refractive index nd of not less than 1.87 and an Abbé number nud of not less than 35 but less than 40.
Patent References 2 to 8 are examples of publications disclosing optical glasses having refractive indexes nd of not less than 1.87 and Abbé numbers nud of not less than 35 but less than 40 and optical glasses having physical properties (refractive indexes nd and Abbé numbers) that closely approximate these values.
To capitalize on the merits of the above-mentioned precision press molding techniques, it is desirable to directly produce from glass melt a glass material—known as a preform—for press molding. In this method, called preform hot forming, a glass melt is caused to flow out, glass melt gobs corresponding in weight to single preforms are successively separated, and the glass melt gobs obtained are formed into preforms having smooth outer surfaces in a cooling step. Accordingly, this method affords advantageous characteristics in that the glass use rate is higher than in methods in which relatively large glass blocks are formed from glass melt and then cut, ground, and polished; no glass scraps are produced during processing; and no processing time or cost is incurred.
However, in the hot forming method, a glass melt gob of weight corresponding to a single preform must be accurately separated and formed into a preform without defects such as devitrification and striae. Accordingly, glass having good stability over a high temperature range is required for hot forming.
When the refractive index nd is raised while maintaining an Abbé number nud of greater than a certain value, there is a strong tendency for the glass to crystallize, ultimately compromising vitrification. When even lower temperature softening properties are imparted to glasses employed in precision press molding, the stability of the glasses tends to decrease. Accordingly, it is difficult to realize glass stability at a level permitting preform hot forming while imparting low-temperature softening properties suited to precision press molding to glasses having a high refractive index and low dispersion.
Patent References 2 to 7 are examples of publications disclosing optical glasses having refractive indexes nd of not less than 1.87 and Abbé numbers nud of not less than 35 but less than 40, and optical glasses having physical properties (refractive indexes nd and Abbé numbers nud) approximating these values, as prerequisites for use as preforms in precision press molding. Patent Reference 8 is an example of a publication disclosing an optical glass that, although not anticipated for use as a preform in precision press molding, has a refractive index nd of not less than 1.87 and an Abbé number nud of not less than 35 but less than 40.
In terms of their refractive indexes nd and/or their Abbé numbers, the optical glasses of Patent References 3 to 7 fail to achieve the above-stated physical properties. The optical glass described in Patent Reference 2 has both a refractive index nd and an Abbé number nud that attain the above-stated physical properties. However, the optical glass described in Patent Reference 2, as will be set forth further below, has a low viscosity at liquid phase temperature; in methods of obtaining preforms by causing a glass melt to flow out and successively separating glass melts gobs of weight corresponding to single preforms, there is a problem in that it can be difficult to form a glass melt gob of weight corresponding to a single preform.
Patent Reference 8 is a publication predating the practice of precision press molding techniques. The optical glass described therein is not anticipated for use as a preform in precision press molding. Thus, Patent Reference 8 does not give the glass transition temperature or liquid phase temperature of the optical glass. Thus, a determination of whether or not use as a preform in precision press molding is possible cannot be made based on the description given in the publication alone. As an example, the present inventors produced the glass described in Embodiment 5 of Patent Reference 8, but glass stability was poor and the glass melt devitrified during stirring, precluding vitrification.
Accordingly, the present invention, devised to solve the above-stated problems, has for its object to provide an optical glass having a refractive index nd of not less than 1.87 and an Abbé number nud of not less than 35 but less than 40; possessing a viscosity temperature dependency permitting the ready forming of preforms for precision press molding; affording good glass stability; and having low temperature melting properties permitting precision press molding with good productivity. Further objects of the present invention are to provide a method for manufacturing preforms for precision press molding from this glass and a method for manufacturing an optical element comprised of this glass.